The present invention relates to a mobile device for generating acoustic shear waves. More particularly the invention relates to a device for generating acoustic shear waves for land seismic prospecting operations.
A prior art device adapted to generate acoustic shear waves is known which comprises essentially a target-member provided with a coupling surface for coupling the device to the earth and a mass for striking the target-member along a direction substantially parallel to the surface of the earth. The mass is connected to a means for imparting thereto a velocity of predetermined amplitude having a component parallel to the coupling surface and whose direction is adjustable with respect to the target-member. Such means comprises, for example, an arm supporting the mass at one of its ends and adapted to pivot at its opposite end with respect to the frame of a carrying vehicle, upon actuation by motor means.
The target-member is suspended from articulated arms on the carrying vehicle and adapted to be vertically displaced from a transportation position to a working position where it is in contact with the earth.
According to an alternative embodiment of the prior art device, the target-member is provided with tracks for the rear wheels of the carrying vehicle. When it is desired to couple the target-member with the earth, the target-member is laid down on the earth at the selected location and it is disconnected from the articulated arms which support it in transportation position. Then the vehicle is moved so that its rear part, consisting in this case of two axles and the wheels mounted thereon, is placed on the target-member and maintained fixed in this position. The mass is then released and strikes the target-member with enhanced efficiency, as far as the seismic energy transmitted to the earth is concerned, because the rear part of the vehicle bears on the tracks.
Such a device has however the disadvantage that each "shot" must be preceded with moving of the vehicle for coupling the target-member with the earth, thereby limiting the maximum repeating frequency of the successive impacts.